1. Field of the Invention
The present invention relates in general to power supply circuits and related components, and is particularly directed to an arrangement for synchronizing a plurality of synthetic ripple generators that generate artificial or synthesized ripple waveforms to control switching operations of a multiphase DC-DC synthetic ripple regulator.
2. Description of the Related Art
Electrical power for integrated circuits is typically supplied by one or more direct current (DC) power sources. In a number of applications the circuit may require multiple regulated voltages that are different from the available supply voltage (which may be relatively low e.g., on the order of a few volts or less, particularly where low current consumption is desirable, such as in portable, battery-powered devices). Moreover, in many applications the load current may vary over several orders of magnitude. To address these requirements it has been common practice to employ pulse or ripple-based regulators, such as a hysteresis or ‘bang-bang’ regulator.
Such a ripple-based DC-DC voltage regulator employs a relatively simple control mechanism and provides a fast response to a load transient. The switching of the ripple regulator is asynchronous, which is advantageous in applications where direct control of the switching edges is desired. For this purpose, a ripple regulator typically employs a hysteresis comparator or the like that controls a gate drive circuit coupled to the control or gate drive inputs of a pair of electronic power switching devices, such as FETs or MOSFETS or the like. The gate drive circuit controllably switches or turns the switching devices on and off in accordance with a pulse width modulation (PWM) switching waveform as known to those skilled in the art.
In such a hysteretic or ‘bang-bang’ regulator, the output PWM signal waveform produced by hysteresis comparator transitions to a first state (e.g., goes high) when the output voltage falls below a reference voltage minus the comparator's inherent hysteresis voltage and the comparator's PWM output transitions to a second state (e.g., goes low) when the output voltage exceeds the reference voltage plus the hysteresis voltage. The application of or increase in load causes the output voltage to decrease below the reference voltage, in response to which the comparator triggers the gate drive to turn on the upper switching device. Because the regulator is asynchronous, the gate drive control signal does not wait for a synchronizing clock, as is common in most fixed frequency PWM control schemes.
Principal concerns with this type of ripple regulator include large ripple voltage, DC voltage accuracy, and switching frequency. Since the hysteretic comparator directly sets the magnitude of the ripple voltage, employing a smaller hysteresis voltage reduces the power conversion efficiency, as switching frequency increases with smaller hysteresis. In order to control the DC output voltage, which is a function of the ripple wave shape, the peaks and valleys of the output ripple voltage is regulated. The DC value of the output voltage is a function of the PWM duty factor. The output voltage wave shape also changes at light loads, when current through the output inductor becomes discontinuous, producing relatively short ‘spikes’ between which are relatively long periods of low voltage. Since the ripple voltage wave shape varies with input line and load conditions, maintaining tight DC regulation is difficult.
In addition, improvements in capacitor technology changes the ripple wave shape. In particular, the current state of ceramic capacitor technology has enabled the equivalent series resistance or ESR (which produces the piecewise linear or triangular wave shape of the output voltage waveform) of ceramic capacitors to be reduced to very low values. At very low values of ESR, however, the output voltage's ripple shape changes from triangular to a non-linear shape (e.g., parabolic and sinusoidal). This causes the output voltage to overshoot the hysteretic threshold, and results in higher peak-to-peak ripple. As a result, the very improvements that were intended to lower the output voltage ripple in DC-DC regulators can actually cause increased ripple when used in a ripple regulator.
A first prior disclosure incorporated herein by reference introduced a synthetic ripple regulator that includes a synthetic ripple voltage generator. As described therein, the generator creates an auxiliary or “synthetic” ripple voltage that effectively replicates the ripple current through the output inductor. The regulator uses the synthetically generated ripple voltage to control toggling of a hysteretic comparator to develop the pulse width modulation (PWM) signal that controls switching of the regulator. In a non-limiting implementation, a transconductance amplifier monitors the phase node voltage of the inductor and supplies an inductor voltage-representative current to a ripple capacitor, which produces the synthetic ripple voltage. Using the replicated inductor current for ripple regulation results in low output ripple, input voltage feed forward, and simplified compensation.
A second prior disclosure incorporated herein by reference introduced a multiphase ripple voltage regulator which employed a hysteretic comparator referenced to upper and lower voltage thresholds. As described therein, the hysteretic comparator monitors a master ripple voltage waveform developed across a capacitor supplied with a current based on the difference between the output voltage and either the input voltage or ground. The output of the hysteretic comparator generates a master clock signal that is sequentially coupled to PWM latches, the states of which define the durations of respective components of the synthesized ripple voltage. A respective PWM latch has a first state initiated by a selected master clock signal and terminated by an associated-phase voltage comparator that monitors a respective-phase synthesized voltage. The present disclosure concerns improvements and/or variations of the multiphase synthetic ripple voltage regulator.